Various types of auto-stereoscopic display device are known. Essentially, such displays include an optical arrangement which directs different views into different directions at which the eyes of a user may be located, so that the user does not need to wear special glasses for this purpose.
The different directions of the views to different locations can for example be achieved with a lens arrangement, with individual lenses covering groups of pixels. The lenses then direct light from different pixels in different directions, and in this way different images (e.g. left and right) can be sent to different directions.
Another approach is to use a barrier arrangement. This again has the function of limiting the direction in which light from individual pixels can be sent.
A problem with this arrangement is that the resolution of the individual views is reduced. By operating in time-multiplex as well as spatial-multiplex manner increased resolution can be obtained.
The designs outlined above provide the different views in fixed different directions. However, other designs provide tracking of the position the eyes of one or more viewers. This means that for each viewer only two views need to be generated, instead of a full set of views (typically 9 or 15) filling the field of view.
This invention is particularly concerned with a known type of auto-stereoscopic display in which a light-steering backlight is used to direct light into the eyes of one or multiple viewers that are detected using cameras. The left- and right-images are displayed sequentially on an LCD.
For each viewer, two direction light source configurations of the backlight are switched on and off in phase with the LCD such that at a given moment in time each eye sees either the correct image or a black screen.
A light-steering backlight can for example comprise a pixellated backlight and a lens arrangement. By controlling backlight elements, the resulting light output direction is determined by the lens. Electrowetting prism arrays can also be provided for directing a backlight output in different directions.
Typically, a lens array is used to achieve the required light-steering. However, these lens arrays suffer from aberrations off-axis. As a consequence, loss of angular resolution occurs at larger incidence angles. This is a problem with backlight-steering approaches generally.
To address this problem, it has for example been proposed to place the illumination sources on a curved surface and to limit the aperture of each optical element. Multiple optical elements are then stacked and shifted a distance horizontally equal to the width of the aperture opening. A vertical diffuser can be used to achieve a homogeneously lit display. This however results in a more complicated optical structure.